1. Field of the Invention
The present invention relates generally to atomic force microscopes (AFMs) and in particular to a Caliper AFM for nanometer-scale length measurements.
2. Description of Related Art
Conventional atomic force microscope (AFM) probes have been developed to perform nanometer-scale measurements on test surfaces, but while those solved many problems, significant problems remained. For example, calibration of such probes is difficult. Great effort must be expended to characterize probe shape in the presence of tip wear, estimate the tip-surface force profile in the presence of surface contamination and variable material composition, and calibrate the scanning stages and other electro-optical sensors and actuators in the presence of manufacturing defects and environmental noise.
In other countries, the only similar caliper work that we are aware of is at the Physikalische Technische Bundesantstalt (PTB), the counterpart of NIST in Germany. This work involves caliper type measurement on a much larger scale. Two opposed scanning tunneling probes are being used to measure the lengths of gauge blocks. One alternative way of obtaining CD measurements with substantial probe modeling is to section the wafer and obtain a cross-section SEM image. However, even on thinned sections the edge resolution of SEMs is limited by the beam-sample interaction. A second and related approach is to manufacture thinned wafer sections that can be measured in a transmission electron microscope. The edge resolution of this approach is extremely good. However, the scale calibration of TEM relies on comparison of the measured images with calibrated artifacts, such as atomic lattice spacings, under nearly identical imaging conditions and is difficult to carry out with high precision. Furthermore, both of these techniques are destructive and involve off-line measurements in vacuum.